Investigation of the Effect of Magnesium on the Microstructure and Mechanical Properties of NiTi Shape Memory Alloy Prep
- PDF / 2,407,625 Bytes
- 11 Pages / 593.972 x 792 pts Page_size
- 57 Downloads / 195 Views
CTION
THE Ni-Ti-based shape memory alloys are the most widely studied and well-known shape memory alloys due to their interesting properties such as pseudoplasticity, superelasticity, and excellent corrosion resistance. The shape memory effect is based on the transformation between austenite and martensite. In this system, the austenite has a cubic B2 lattice, while the crystal structure of martensite is monoclinic. Superelasticity is effective when the material can sustain a large deformation without permanent strain. The stress-induced martensite occurs during applied stress. After unloading, it returns to the austenite structure when deforming above the transformation temperature. This is the most important property of this alloy for industrial applications.[1,2] NiTi alloys are usually industrially produced by melting processes, which include vacuum induction melting (VIM) and vacuum arc remelting (VAR). The
ANDREA SˇKOLA´KOVA´, PAVEL NOVA´K, PAVEL SALVETR, HYNEK MORAVEC, and VA´CLAV SˇEFL are with the Department of Metals and Corrosion Engineering, University of Chemistry and Technology, Prague, 166 28 Prague 6, Czech Republic. Contact email: [email protected] DAVY DEDUYTSCHE and CHRISTOPHE DETAVERNIER are with the Department of Solid State Sciences, Ghent University, Krijgslaan 281 S1, 9000 Gent, Belgium. Manuscript submitted February 8, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS A
VIM method produces homogeneous ingots, but there is a possibility of contamination of the melt due to the high reactivity of molten titanium. Titanium has a high affinity to oxygen and carbon. Therefore, contamination by oxygen or carbon can significantly affect the properties of this alloy. For this reason, bulk or coated crucibles made of zirconia or yttria are used. However, this material also contaminates the molten NiTi and, moreover, produces oxide inclusions. Furthermore, it has been discovered that Y2O3 is better than ZrO2, but yttria’s disadvantage is its price. The VAR method produces the inhomogeneous product without impurities; therefore, this process has to be repeated several times (at least 4 times) to ensure the required structural homogeneity. For this reason, self-propagating high-temperature synthesis (SHS) can be one of the possibilities of how to prepare highly pure NiTi alloy. The SHS process has been studied extensively for the fabrication of intermetallic compounds for several years.[3,4] SHS is a method of synthesizing many intermetallic compounds within short processing times at high reaction temperatures.[5] Compared with melting methods, the advantages of SHS are in the high purity of the products, energy savings, short process time, and reduced processing cost. Homogeneous alloys with accurate stoichiometry can be synthesized by this route. The process is completed in less than 20 minutes. Moreover, porous alloys can be produced, having their
microstructure similar to the human bone. Even samples with 40 to 60 pct porosity can be obtained.[5–8] The mechanism of sintering has been investigated recen
Data Loading...
